Speedy setter, delightful guy primarily filling in clues (9)
I believe the answer is:
superglue
'speedy setter' is the definition.
(superglue sets or dries quickly)
'delightful guy primarily filling in clues' is the wordplay.
'delightful' becomes 'super' (I've seen this before).
'primarily' indicates taking the first letters.
'filling in' says to take the centre (the content or filling of the word).
The centre of 'clues' is 'lue'.
The first letter of 'guy' is 'g'.
'super'+'g'+'lue'='SUPERGLUE'
(Other definitions for superglue that I've seen before include "Extra strong adhesive" , "Substance used to stick" , "Gripper" , "Fast-bonding substance" , "that'll fix a lot of things!" .)